The present invention relates to optical substrates, particularly to optical substrates that contain coated reflective optics, and more particularly to a pedestal type optical substrate that maintains acceptable figure before or after coating, is not sensitive to mount disturbance loads, and demonstrates high dynamic stiffness.
Optics for extreme ultraviolet (EUV) lithography imaging systems, for example, require coated reflective optics that are precisely figured at the angstrom level. In such reflective optics, the optical substrates are first figured, then coated with a film of material that is reflective in the ultraviolet wavelengths. Unfortunately, the coated film has a residual stress that causes the substrate to deform from its desired figure. It has been shown that in order to reduce stress-induced non-spherical deformation on the figured optical substrate it is necessary to 1) maintain thickness uniformity of the optical substrate; 2) maximize "freeboard", i.e., that portion of an optical surface between the clear aperture and the edge of the optical substrate; and 3) minimize the thickness of the optical substrate. Reducing substrate thickness reduces non-spherical deformation near the edge of the substrate and is particularly important when freeboard is minimal. It must be noted that substrate thickness cannot be reduced without bound. Extremely thin substrates are susceptible to non-spherical deformation induced by spatial variations in the magnitude of the film stress. Thus, for a disk-like optical substrate, the non-spherical component of the figure distortion decreases as the aspect ratio (width divided by height) of the substrate increases. Thus, while thinner substrates deform more due to the coating, the deformation is primarily spherical. In many cases an imaging system is relatively insensitive to spherical changes in an individual optic. Thus, a thinner substrate reduces the negative effect of coating residual stress.
The coated optics must be assembled into the imaging system using some form of mechanical attachment. The mechanical attachment, referred to as the optical mount, is physically attached to the substrate. This attachment is a mechanism through which undesired figure distorting forces and moment are applied to the substrate. There are many sources of these disturbance inputs, including system temperature changes, distortion of their imaging system structure, and residual stress due to the mounting process. The sensitivity of the optics surface figure is directly related to the mechanical stiffness of the substrate. An infinitely stiff substrate would not deform due to these inputs. A realistic, disk-like substrate will always demonstrate a level of sensitivity to these mount induced disturbance loads. The lower the aspect ratio of a disk-like substrate, the lower the optic's sensitivity to disturbance loads. Thus, a thicker substrate reduces the sensitivity of a substrate to disturbance loads.
Frequently, there is a requirement that an optical substrate demonstrates a high degree of dynamic stiffness. A low aspect ratio substrate generally provides for more favorable (higher) dynamic stiffness.
The problem is that while a thinner substrate reduces negative coating effects, it may not provide the necessary stiffness and insensitivity to mount disturbances. The present invention, which involves a pedestal substrate, addresses the need to have an optical substrate that maintains acceptable figure before or after coating, is not sensitive to mount disturbance loads, and demonstrates high dynamic stiffness. The pedestal substrate of the present invention comprises the basic components: 1) a disk-like optic or substrate section, the top surface of which is to be coated with reflective material, 2) a disk-like base section, and 3) a connecting section between the base and optic or substrate sections. These three sections may be formed as a monolith from a solid piece of material, the optic or substrate section and the connecting section may be formed from a solid piece, or the three sections formed individually and secured together. The pedestal substrate approach of the present invention permits the optic component designer to independently control the effects of residue coating film stress, mount disturbance loads, and substrate dynamic stiffness.
One aspect of the present invention is to overcome the problem of mounting torque created by the thinner optical substrate or section, shown to be particularly desirable by the design rules set forth herein. The optical substrate can include a coated optic section or mirror; the terms "optic section" and "mirror" as used here are synonymous. Thus, a second aspect of the present invention is to substantiate the quantitative design rules that have been established for reducing highly non-spherical deformation that arises in the coated optic section, which cannot be corrected for, due to stresses induced by the multilayer reflective film coating. These design rules, which establish and quantify the relationship between the thickness of the optic section and its diameter and freeboard, show that it is particularly desirable to maximize the freeboard surrounding the clear aperture because the majority on non-spherical deformation occurs near the edge of the optic section. It should be noted here that stopping the multilayer reflective coating short (i.e., several millimeters) of the edge of the optic section results in only a small reduction in the edge deformation. As discussed above, minimizing the aspect ratio (ratio of height, or thickness, to width, or diameter) of the optic section is also desirable because this enhances spherical deformation, which can be corrected for, at the expense of non-spherical deformation, which cannot. Moreover, it is also desirable to minimize thickness variation across the optic section (between front and back surfaces) since thickness variation also results in non-correctable non-spherical deformation.